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Abstract:

An image generating method is provided. The method includes performing a
rearrangement process and an interpolation process on fan beam projection
data, the fan beam projection data acquired by a scan that includes
rotating a radiation source and a detector having a plurality of
detecting elements arranged in a channel direction, wherein the
interpolation process generates equally-spaced parallel beam projection
data in which channel-direction intervals are equal therebetween, and
wherein the interpolation process is performed with respect to a
plurality of view directions. The method further includes performing a
back-projection process on the equally-spaced parallel beam projection
data to thereby reconstruct an image, wherein the channel-direction
intervals between the equally-spaced parallel beam projection data are
smaller than a reference interval obtained by dividing an interval
between the detecting elements in the channel direction by a projection
enlargement rate.

Claims:

1. An image generating method comprising the steps: performing a
rearrangement process and an interpolation process on fan beam projection
data for a plurality of views, the fan beam projection data acquired by a
scan that includes rotating a radiation source and a detector having a
plurality of detecting elements arranged in a channel direction about a
target, wherein the interpolation process generates equally-spaced
parallel beam projection data in which channel-direction intervals are
equal therebetween, and wherein the interpolation process is performed
with respect to a plurality of view directions; and performing a
back-projection process on the equally-spaced parallel beam projection
data to thereby reconstruct an image, wherein the channel-direction
intervals between the equally-spaced parallel beam projection data are
smaller than a reference interval obtained by dividing an interval
between the detecting elements in the channel direction by a projection
enlargement rate at a detection surface of the detector when a center of
rotation of each of the radiation source and the detector is set as a
reference.

2. The image generating method according to claim 1, wherein the interval
between the equally-spaced parallel beam projection data in the channel
direction is 1/N (where N is an integer greater than or equal to 2) of
the reference interval.

3. The image generating method according to claim 2, wherein N is any of
integer ranging from 2 to 4.

4. The image generating method according to claim 2, wherein, near the
center of rotation, a position of each of the equally-spaced parallel
beam projection data in the channel direction substantially overlaps with
a position in the channel direction of each of unequally-spaced parallel
beam projection data generated by the rearrangement process.

5. The image generating method according to claim 1, wherein an order of
the interpolation process is changed according to a distance of data
attempted to be generated by the interpolation process from the center of
rotation.

6. The image generating method according to claim 1, wherein a weighting
of original data used in the interpolation process is changed according
to a distance between a position of data attempted to be generated by the
interpolation process and a position of the original data.

7. An image generating apparatus comprising: a data converting unit
configured to perform a rearrangement process and an interpolation
process on fan beam projection data for a plurality of views, the fan
beam projection data acquired by a scan that includes rotating a
radiation source and a detector having a plurality of detecting elements
arranged in a channel direction about a target, wherein the interpolation
process generates equally-spaced parallel beam projection data in which
channel-direction intervals are equal therebetween, and wherein the
interpolation process is performed with respect to a plurality of view
directions; and an image reconstructing unit configured to perform a
back-projection process on the equally-spaced parallel beam projection
data to thereby reconstruct an image, wherein the channel-direction
intervals between the equally-spaced parallel beam projection data are
smaller than a reference interval obtained by dividing an interval of
between the detecting elements in the channel direction by a projection
enlargement rate at a detection surface of the detector when a center of
rotation of each of the radiation source and the detector is set as a
reference.

8. The image generating apparatus according to claim 7, wherein the
interval between the equally-spaced parallel beam projection data in the
channel direction is 1/N (where N is an integer greater than or equal to
2) of the reference interval.

9. The image generating apparatus according to claim 8, wherein N is any
integer ranging from 2 to 4.

10. The image generating apparatus according to claim 8, wherein, near
the center of rotation, a position of each of the equally-spaced parallel
beam projection data in the channel direction substantially overlaps with
a position in the channel direction of each of unequally-spaced parallel
beam projection data generated by the rearrangement process.

11. The image generating apparatus according to claim 7, wherein the data
converting unit is configured to change an order of the interpolation
process according to a distance of data attempted to be generated by the
interpolation process from the center of rotation.

12. The image generating apparatus according to claim 7, wherein the data
converting unit is configured to change a weighting of original data used
in the interpolation process according to a distance between a position
of data attempted to be generated by the interpolation process and a
position of the original data.

13. A radiation tomographic imaging apparatus comprising: a radiation
source; a detector having a plurality of detecting elements arranged in a
channel direction; a data acquiring unit configured to acquire fan beam
projection data for a plurality of views during a scan that includes
rotating the radiation source and the detector about a target; a data
converting unit configured to perform a rearrangement process and an
interpolation process on the acquired fan beam projection data to
generate equally-spaced parallel beam projection data in which
channel-direction intervals are equal therebetween, the interpolation
process performed with respect to a plurality of view directions; and an
image reconstructing unit configured to perform a back-projection process
on the equally-spaced parallel beam projection data to thereby
reconstruct an image, wherein the channel-direction intervals between the
equally-spaced parallel beam projection data are smaller than a reference
interval obtained by dividing an interval between the detecting elements
in the channel direction by a projection enlargement rate at a detection
surface of the detector when a center of rotation of each of the
radiation source and the detector is set as a reference.

14. The radiation tomographic imaging apparatus according to claim 13,
wherein the interval between the equally-spaced parallel beam projection
data in the channel direction is 1/N (where N is an integer greater than
or equal to 2) of the reference interval.

15. The radiation tomographic imaging apparatus according to claim 14,
wherein N is any integer ranging from 2 to 4.

16. The radiation tomographic imaging apparatus according to claim 14,
wherein, near the center of rotation, a position of each of the
equally-spaced parallel beam projection data in the channel direction
substantially overlaps with a position in the channel direction of each
of unequally-spaced parallel beam projection data generated by the
rearrangement process.

17. The radiation tomographic imaging apparatus according to claim 13,
wherein the data converting unit is configured to change an order of the
interpolation process according to a distance of data attempted to be
generated by the interpolation process from the center of rotation.

18. The radiation tomographic imaging apparatus according to claim 13,
wherein the data converting unit is configured to change a weighting of
original data used in the interpolation process according to a distance
between a position of data attempted to be generated by the interpolation
process and a position of the original data.

19. The radiation tomographic imaging apparatus according to claim 13,
wherein the data acquiring unit is configured to assign at least 1200
views for performing acquisition of actual data to a rotating angle of
each of the radiation source and the detector to thereby acquire the fan
beam projection data for the plurality of views.

20. The radiation tomographic imaging apparatus according to claim 15,
wherein, near the center of rotation, a position of each of the
equally-spaced parallel beam projection data in the channel direction
substantially overlaps with a position in the channel direction of each
of unequally-spaced parallel beam projection data generated by the
rearrangement process.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Japanese Patent Application
No. 2012-57071 filed Mar. 14, 2012, which is hereby incorporated by
reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to an image generating method, an
image generating apparatus and a radiation tomographic imaging apparatus
which fan-para convert radiation projection data and perform back
projection thereon to thereby reconstruct an image, and a program
therefor.

[0003] In a third-generation X-ray CT (Computed Tomography) apparatus, X
rays of a fan beam are used in the acquisition of projection data. There
is a case in which the so-acquired fan beam projection data are converted
to parallel beam projection data, followed by being subjected to a
back-projection process to reconstruct an image.

[0004] This conversion is generally referred to as "fan-para conversion".
An image reconstructing method using the fan-para conversion is called
"fan-para conversion method". The fan-para conversion method is
principally performed with the aim of ensuring the uniformity of CT
values, suppressing artifacts at the time that MPR (Multi-Plane
Reconstruction) is carried out, reducing a computational processing
amount, and so on.

[0005] On the other hand, when fan beam projection data of plural views
are taken apart every data in a channel direction and rearranged in a
simplistic form to acquire parallel beam projection data, the positions
of radiation paths corresponding to the respective data are unequally
spaced in the channel direction.

[0006] Since, however, Fourier transform corresponding to frequency
conversion is performed on the parallel beam projection data after the
fan-para conversion in the fan-para conversion method, the radiation
paths corresponding to the respective data need to be arranged at equal
intervals in the channel direction.

[0007] Thus, normally, when the fan-para conversion is performed, not only
a rearrangement process but also an interpolation process in the channel
direction is performed on the fan beam projection data to acquire
equally-spaced parallel beam projection data in which the positions of
radiation paths are arranged at equal integrals in the channel direction
(refer to, for example, paragraph

[0004] of Japanese Unexamined Patent
Publication No. 2012-005757).

[0008] When, however, the equally-spaced parallel beam projection data
acquired by performing the interpolation process in the channel direction
are compared with pre-interpolation process data (i.e., data before the
interpolation processing), more errors from true values are included
therein, thereby leading to a reduction in the spatial resolution of a
reconstructed image.

[0009] With the foregoing in view, there has been a demand for a
technology that enables a reduction in the spatial resolution of the
reconstructed image to be suppressed even if the fan-para conversion is
performed.

BRIEF DESCRIPTION OF THE INVENTION

[0010] In a first aspect, an image generating method is provided. The
image generating method includes a data conversion step for performing a
rearrangement process and an interpolation process on fan beam projection
data of a plurality of views acquired by a scan for rotating a radiation
source and a detector having a plurality of detecting elements arranged
in a channel direction about a target to thereby acquire equally-spaced
parallel beam projection data in which channel-direction intervals are
equal, with respect to a plurality of view directions, and an image
reconstruction step for performing a back-projection process on the
acquired equally-spaced parallel beam projection data to thereby
reconstruct an image, in which the interval between the equally-spaced
parallel beam projection data in the channel direction is smaller than a
reference interval obtained by dividing the interval of arrangement of
the detecting elements in the channel direction by a projection
enlargement rate at a detection surface of the detector at the time when
the center of rotation of each of the radiation source and the detector
is taken as a reference.

[0011] In a second aspect, the image generating method according to the
first aspect is provided, in which the interval between the
equally-spaced parallel beam projection data in the channel direction is
1/N (where N is an integer greater than or equal to 2) of the reference
interval.

[0012] In a third aspect, the image generating method according to the
second aspect is provided, in which N is any of the integers ranging from
2 to 4.

[0013] In a fourth, aspect the image generating method according to the
second or third aspect is provided, in which the position of each of the
equally-spaced parallel beam projection data in the channel direction is
set to substantially overlap with the position in the channel direction,
of each of unequally-spaced parallel beam projection data obtained by
performing a rearrangement process on the fan beam projection data, in
the neighborhood of the center of rotation.

[0014] In a fifth aspect, the image generating method according to any one
of the first through fourth aspects is provided, in which in the data
conversion step, the order of the interpolation process is changed
according to the distance of data attempted to be acquired by the
interpolation process from the center of rotation.

[0015] In a sixth aspect, the image generating method according to any one
of the first through fourth aspects is provided, in which in the data
conversion step, weighting to the original data used in the interpolation
process is changed according to the distance between the position of the
data attempted to be acquired by the interpolation process and the
position of the original data used in the interpolation process.

[0016] In a seventh aspect, an image generating apparatus is provided. The
image generating apparatus includes a data converting unit for performing
a rearrangement process and an interpolation process on fan beam
projection data of a plurality of views acquired by a scan for rotating a
radiation source and a detector having a plurality of detecting elements
arranged in a channel direction about a target to thereby acquire
equally-spaced parallel beam projection data in which channel-direction
intervals are equal, with respect to a plurality of view directions, and
an image reconstructing unit for performing a back-projection process on
the acquired equally-spaced parallel beam projection data to thereby
reconstruct an image, in which the interval between the equally-spaced
parallel beam projection data in the channel direction is smaller than a
reference interval obtained by dividing the interval of arrangement of
the detecting elements in the channel direction by a projection
enlargement rate at a detection surface of the detector at the time that
the center of rotation of each of the radiation source and the detector
is taken as a reference.

[0017] In an eighth aspect, the image generating apparatus according to
the seventh aspect is provided, in which the interval between the
equally-spaced parallel beam projection data in the channel direction is
1/N (where N is an integer greater than or equal to 2) of the reference
interval.

[0018] In a ninth aspect, the image generating apparatus according to the
eighth aspect is provided, in which N is any of the integers ranging from
2 to 4.

[0019] In a tenth aspect, the image generating apparatus according to the
eighth or ninth aspect is provided, in which the position of each of the
equally-spaced parallel beam projection data in the channel direction is
set to substantially overlap with the position in the channel direction,
of each of unequally-spaced parallel beam projection data obtained by
performing a rearrangement process on the fan beam projection data, in
the neighborhood of the center of rotation.

[0020] In an eleventh aspect, the image generating apparatus according to
any one of the seventh through tenth aspects is provided, in which the
data converting unit changes the order of the interpolation process
according to the distance of data attempted to be acquired by the
interpolation process from the center of rotation.

[0021] In a twelfth aspect, the image generating apparatus according to
any one of the seventh through tenth aspects is provided, in which the
data converting unit changes weighting to the original data used in the
interpolation process according to the distance between the position of
the data attempted to be acquired by the interpolation process and the
position of the original data used in the interpolation process.

[0022] In a thirteenth aspect, a radiation tomographic imaging apparatus
is provided. The radiation tomographic imaging apparatus includes a
radiation source, a detector having a plurality of detecting elements
arranged in a channel direction, a data acquiring unit for acquiring fan
beam projection data of a plurality of views by a scan for rotating the
radiation source and the detector about a target, a data converting unit
for performing a rearrangement process and an interpolation process on
the acquired fan beam projection data to thereby acquire equally-spaced
parallel beam projection data in which channel-direction intervals are
equal, with respect to a plurality of view directions, and an image
reconstructing unit for performing a back-projection process on the
acquired equally-spaced parallel beam projection data to thereby
reconstruct an image, in which the interval between the equally-spaced
parallel beam projection data in the channel direction is smaller than a
reference interval obtained by dividing the interval of arrangement of
the detecting elements in the channel direction by a projection
enlargement rate at a detection surface of the detector at the time that
the center of rotation of each of the radiation source and the detector
is taken as a reference.

[0023] In a fourteenth aspect, the radiation tomographic imaging apparatus
according to the thirteenth aspect is provided, in which the interval
between the equally-spaced parallel beam projection data in the channel
direction is 1/N (where N is an integer greater than or equal to 2) of
the reference interval.

[0024] In a fifteenth aspect, the radiation tomographic imaging apparatus
according to the fourteenth aspect is provided, in which N is any of the
integers ranging from 2 to 4.

[0025] In a sixteenth aspect, the radiation tomographic imaging apparatus
according to the fourteenth or fifteenth aspect is provided, in which the
position of each of the equally-spaced parallel beam projection data in
the channel direction is set to substantially overlap with the position
of each of unequally-spaced parallel beam projection data obtained by
performing a rearrangement process on the fan beam projection data, in
the neighborhood of the center of rotation.

[0026] In a seventeenth aspect, the radiation tomographic imaging
apparatus according to any one of the thirteenth through sixteenth
aspects is provided, in which the data converting unit changes the order
of the interpolation process according to the distance of data attempted
to be acquired by the interpolation process from the center of rotation.

[0027] In an eighteenth aspect, the radiation tomographic imaging
apparatus according to any one of the thirteenth through sixteenth
aspects is provided, in which the data converting unit changes weighting
to the original data used in the interpolation process according to the
distance between the position of the data attempted to be acquired by the
interpolation process and the position of the original data used in the
interpolation process.

[0028] In a nineteenth aspect, the radiation tomographic imaging apparatus
according to any one of the thirteenth through eighteenth aspects is
provided, in which the data acquiring unit assigns views for performing
the acquisition of actual data to a rotating angle per rotation of each
of the radiation source and the detector 1200 more to thereby acquire the
fan beam projection data of the plural views.

[0029] In a twentieth aspect, a program is provided. The program allows a
computer to function as data converting unit for performing a
rearrangement process and an interpolation process on fan beam projection
data of a plurality of views acquired by a scan for rotating a radiation
source and a detector having a plurality of detecting elements arranged
in a channel direction about a target to thereby acquire equally-spaced
parallel beam projection data in which channel-direction intervals are
equal, with respect to a plurality of view directions, and image
reconstructing unit for performing a back-projection process on the
acquired equally-spaced parallel beam projection data to thereby
reconstruct an image, in which the interval between each radiation path
in the channel direction is smaller than a reference interval obtained by
dividing the interval of arrangement of the detecting elements in the
channel direction by a projection enlargement rate at a detection surface
of the detector at the time that the center of rotation of each of the
radiation source and the detector is taken as a reference.

[0030] According to the above aspects, equally-spaced parallel beam
projection data are acquired in such a manner that when fan-para
conversion is performed, the interval between each data in a channel
direction becomes an interval smaller than a reference interval obtained
by dividing the interval of arrangement of detecting elements in the
channel direction by a projection enlargement rate at a detection surface
of a detector at the time that a so-called iso-center is set as a
reference. Therefore, pre-interpolation data high in accuracy or
post-interpolation data (i.e., data after interpolation processing) close
thereto can be more used in a back-projection process. Even if the
fan-para conversion is performed, a reduction in the spatial resolution
of a reconstructed image can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a diagram schematically showing a configuration of an
exemplary X-ray CT apparatus.

[0032]FIG. 2 is a functional block diagram illustrating a configuration
of a section related to an image generating process in the X-ray CT
apparatus.

[0035]FIG. 5 is a diagram for describing an interpolation process on
parallel beam projection data in a channel direction by a conventional
method.

[0036] FIGS. 6A and 6B show graphs which illustrate changes in spatial
resolution corresponding to the distances from an iso-center at a
reconstructed image by a general method and are determined by simulation.

[0037]FIG. 7 is a diagram for describing an interpolation process on
parallel beam projection data in a channel direction by an exemplary
method.

[0038]FIG. 8 is a flow chart illustrating the flow of an exemplary image
generating process.

[0039]FIG. 9 is a diagram showing a first example of comparison between
an image based on a general method and an image based on the exemplary
method.

[0040] FIG. 10 is a diagram showing a second example of comparison between
an image based on the general method and an image based on the exemplary
method.

[0041] FIG. 11 is a diagram showing a third example of comparison between
an image based on the general method and an image based on the exemplary
method.

[0042]FIG. 12 is a diagram showing a fourth example of comparison between
an image based on the general method and an image based on the exemplary
method.

DETAILED DESCRIPTION OF THE INVENTION

[0043] An exemplary embodiment will hereinafter be described.
Incidentally, the disclosure is not limited by or to the exemplary
embodiment.

First Embodiment

[0044]FIG. 1 is a diagram schematically showing a configuration of an
X-ray CT apparatus according to the exemplary embodiment.

[0045] The X-ray CT apparatus 100 is equipped with an operation console 1,
a imaging table 10 and a scan gantry 20.

[0046] The operation console 1 is equipped with an input device 2 which
accepts an input from an operator, a central processing unit 3 which
executes control of respective parts for performing subject's imaging,
data processing for generating an image, etc., a data acquisition buffer
5 which acquires or collects data acquired by the scan gantry 20, a
monitor 6 which displays each image thereon, and a storage device 7 which
stores programs, data, etc. therein.

[0047] The imaging table 10 is equipped with a cradle 12 which conveys a
subject 40 to a cavity portion B of the scan gantry 20 with the subject
40 placed thereon. The cradle 12 is elevated and linearly moved
horizontally by a motor built in the imaging table 10. Incidentally, in
the exemplary embodiment, the direction of a body axis of the subject 40,
i.e., the horizontal linear moving direction of the cradle 12 is assumed
to be a z direction, its vertical direction is assumed to be a y
direction, and its horizontal direction orthogonal to the z and y
directions is assumed to be an x direction.

[0048] The scan gantry 20 has a rotating section 15 and a body section 20a
which rotatably supports the rotating section 15. The rotating section 15
is provided with an X-ray tube 21, an X-ray controller 22 which controls
the X-ray tube 21, an aperture 23 which shapes X-rays 81 generated from
the X-ray tube 21 into a fan beam or a cone beam, an X-ray detector 24
which detects the X-rays 81 penetrated through the subject 40, and a
rotating section controller 26 which controls the X-ray controller 22 and
the aperture 23. The body section 20a is equipped with a control
controller 29 which performs communication of control signals or the like
with the operation console 1 and the imaging table 10. The rotating
section 15 and the body section 20a are electrically coupled to each
other via a slip ring 30.

[0049] The X-ray tube 21 and the X-ray detector 24 are disposed opposite
to each other with an imaging space in which the subject 40 is placed,
i.e., the cavity portion B of the scan gantry 20 interposed therebetween.
When the rotating section 15 is rotated, the X-ray tube 21 and the X-ray
detector 24 are rotated about the subject 40 while their positional
relation is being maintained. The X-rays 81 shown in the form of the fan
beam or cone beam, which are radiated from the X-ray tube 21 and shaped
by the aperture 23, penetrate the subject 40 and are applied onto a
detection surface of the X-ray detector 24.

[0050] Incidentally, here, the direction of expansion of the X-rays 18
shown in the form of this fan beam or cone beam at an xy plane is assumed
to be shown as a channel direction (CH direction), the direction of
expansion thereof in the z direction or the z direction itself is assumed
to be shown as a slice direction (SL direction), and the direction
thereof to the center of rotation of the rotating section 15 at the xy
plane is assumed to be shown as an iso-center direction (I direction).

[0051] The X-ray detector 24 includes a plurality of detecting elements
24i arranged in the channel and slice directions. Incidentally, the
number of the detecting elements in the channel direction is, for
example, about 900 within an angular range of 60°. The interval of
arrangement of the detecting element is, for example, about 1 mm.

[0055] The data acquisition unit 31 controls the scan gantry 20 to run a
scan, thereby acquiring fan beam projection data of plural views. The fan
beam projection data are of projection data in which X-ray paths
corresponding to respective data are expanded in fan-beam form, i.e.,
radially in a predetermined angular range.

[0056] In the exemplary embodiment, as shown in FIG. 3, the above scan is
performed while evenly allocating a predetermined number of views to a
turning or rotating angle corresponding to one rotation in such a manner
that a rotating angle corresponding to one view becomes substantially
equal to a rotating angle Δα corresponding to the interval of
arrangement of the detecting elements in the channel direction.

[0057] Incidentally, the fan beam projection data of the respective views
may all be actual data based directly on detected signals of the
detecting elements 24i and not subjected to an interpolation process or
the like. Some of the fan beam projection data may be acquired by
interpolation of actual data in a view direction. When the assignment of
views at which actual data are acquired, is however made excessively
rough depending greatly on the interpolation in the view direction, it
exerts a bad influence on the spatial resolution of a reconstructed
image. Therefore, the number of views which are assigned to the turning
angle corresponding to one rotation and at which the acquisition of
actual data is performed, is set to, for example, at least 800 views,
1200 views or more, or 1600 views or more.

[0058] FIGS. 4A and 4B show geometries at fan-para conversion, of which
FIG. 4A indicates the geometry of a fan beam and the FIG. 4B indicates
the geometry of a parallel beam, respectively.

[0060] The rearrangement process will first be explained. The
rearrangement process is a process in which fan beam projection data of
plural views are taken apart every data for their rearrangements to
thereby acquire parallel beam projection data in which X-ray paths are
parallel, with respect to a plurality of view directions.

[0061] As is understood from FIGS. 4A and 4B, in the parallel beam
projection data acquired after the rearrangement process, the distance Da
from the iso-center (the center of rotation of the scan gantry) of an
X-ray path corresponding to each data is expressed in
Dα=Fi×sin (α). Here, Fi indicates the distance between
an X-ray focal point and the iso-center ISO, and a indicates the rotating
angle of each detecting element 24i. That is, the parallel beam
projection data acquired after the rearrangement process are
unequally-spaced parallel beam projection data in which X-ray paths
corresponding to respective data are arranged at unequal intervals in the
channel direction.

[0062] As described above, the scan executed by the data acquisition unit
31 is performed while evenly assigning the predetermined number of views
to the rotating angle corresponding to one rotation in such a manner that
the rotating angle corresponding to one view becomes substantially equal
to the rotating angle Δα corresponding to the interval of
arrangement of the detecting elements 24i in the channel direction.
Therefore, the channel-direction interval between the X-ray paths
corresponding to the unequally-spaced parallel beam projection data
assumes, in the neighborhood of the iso-center ISO, an interval Ad
obtained by dividing the interval of arrangement of the detecting
elements 24i of the X-ray detector 24 in the channel direction by a
projection enlargement rate (also called an X-ray enlargement rate) at
the detection surface taken when the iso-center ISO is set as the
reference. That is, when a straight line that connects via the
neighborhood of the iso-center ISO from an X-ray focal point 21f to the
center of one given detecting element 24i, and a straight line that
connects from the X-ray focal point 21f to a detecting element 24i
adjacent to the given detecting element 24i are assumed to exist as shown
in FIG. 3, the interval Δd becomes equivalent to the distance
between the two straight lines in the neighborhood of the iso-center ISO.
The spatial resolution of the reconstructed image cannot be rendered
higher than the interval Δd from a geometric point of view. This
interval Δd has been considered to be a limit condition that
increases the spatial resolution of the reconstructed image to the
maximum. Here, the interval Δd is assumed to be referred to as a
"reference interval". Incidentally, when the interval of arrangement of
the detecting elements in the channel direction is about 1 mm, the
reference interval Δd is about 0.5 mm, for example.

[0063] The interpolation process in the channel direction will next be
explained. The interpolation process in the channel direction is a
process in which an interpolation process is performed on the
unequally-spaced parallel beam projection data obtained by the
rearrangement process to thereby acquire equally-spaced parallel beam
projection data in which X-ray paths corresponding to the respective data
are parallel and arranged at equal intervals in the channel direction.
Incidentally, upon the interpolation process, the position of each of the
X-ray paths at the equally-spaced parallel beam projection data is set in
such a manner as to substantially overlap with each of the X-ray paths at
the unequally-spaced parallel beam projection data in the neighborhood of
the iso-center.

[0064] The interpolation process in the channel direction will now be
explained while making a comparison between a general method and a method
based on the exemplary embodiment.

[0065]FIG. 5 is a diagram for describing the interpolation process in the
channel direction by the general method. A group of arrows
(non-normalized Fan Data) on the upper side of FIG. 5 is expressed in the
form of simplification of X-ray paths at unequally-spaced parallel beam
projection data P1. A group of arrows (normalized Fan Data) on the lower
side of FIG. 5 is expressed in the form of simplification of X-ray paths
at equally-spaced parallel beam projection data P2 after the
interpolation process by the general method.

[0066] In the general method, as shown in FIG. 5, the interval between the
X-ray paths in the channel direction at the equally-spaced parallel beam
projection data P2 is set to be substantially equal to the reference
interval Δd. That is, the set of the interval between the X-ray
paths at the equally-spaced parallel beam projection data P2 to be
acquired, i.e., the sampling interval in the channel direction has
already reached such an upper limit that the spatial resolution of the
reconstructed image can be rendered highest. Even if sampling is done
finer than it, a computational processing amount merely increases and
hence such a set as considered not to contribute to an improvement in the
spatial resolution is made.

[0067] The difference between the interval between the X-ray paths at the
unequally-spaced parallel beam projection data P1 and the interval
between the X-ray paths at the equally-parallel beam projection data P2
is actually small. A positional relationship between each X-ray path at
the unequally-spaced parallel beam projection data P1 and each X-ray path
at the equally-spaced parallel beam projection data P2 is however held
consistent in the neighborhood of the iso-center ISO, and small
displacements begin to take place as the distance from the iso-center ISO
increases. With the increase in the distance from the iso-center ISO, the
small displacements pile up and gradually increase, thus assuming a local
maximum point at a given position. Information necessary to maintain the
spatial resolution is lost around the local maximum point. After the
local maximum point, the positional displacement of each X-ray path
gradually decreases and does not occur in a given position, so that the
X-ray paths coincide with each other. Thereafter, the displacement
gradually increases and assumes a local maximum point again.

[0068] Thus, the spatial resolution, e.g., having a MTF (Modulation
Transfer Function) as its index, becomes lower with the periodicity
according to the distance from the iso-center ISO.

[0069] FIGS. 6A and 6B illustrate graphs which show changes in spatial
resolution corresponding to the distances from the iso-center ISO in a
reconstructed image by a general method and are obtained by simulation.
The graph of FIG. 6A is obtained by determining the number of line pairs
per cm taken when an MTF value becomes 50%, at respective positions on an
image and plotting them. The graph of FIG. 6B is obtained by determining
the number of line pairs per cm taken when an MTF value becomes 10%, at
respective positions on an image and plotting them. Curves of changes in
the spatial resolution of a reconstructed image based on fan beam
projection data are placed even at both graphs for reference. Even in
these graphs, the manner in which the spatial resolution is reduced with
periodicity according to the distance from the iso center ISO is observed
at the reconstructed image based on the general method.

[0070] In the case of the general method, the amount of positional
displacement between the X-ray path at the unequally-spaced parallel beam
projection data and the X-ray path at the equally-spaced parallel beam
projection data becomes partly large so that the spatial resolution is
reduced therearound.

[0071]FIG. 7 is a diagram for describing the interpolation process in the
channel direction by the method of the exemplary embodiment. A group of
arrows on the upper side of FIG. 7 is represented in the simplified form
of X-ray paths at unequally-spaced parallel beam projection data P1. A
group of arrows on the lower side of FIG. 7 is represented in simplified
form of X-ray paths at equally-spaced parallel beam projection data P3
after the interpolation process by the method of the exemplary
embodiment.

[0072] In the method of the exemplary embodiment, as shown in FIG. 7, the
interval between the X-ray paths at the equally-spaced parallel beam
projection data P3 in the channel direction is set smaller than a
reference interval Δd. Further, the interval therebetween is set to
be substantially the same as 1/N (where N: integer greater than or equal
to 2) of the reference interval Δd.

[0073] Seemingly, even if done in this manner, this might not seem like a
contribution to an improvement in the spatial resolution. In fact,
however, when the interval between the X-ray paths at the equally-spaced
parallel beam projection data P3 in the channel direction is set smaller
than the reference interval Ad in this way, the opportunity that
pre-interpolation data high in accuracy (i.e., each data itself at the
unequally-spaced parallel beam projection data P1 or post-interpolation
data close thereto) is used in a back-projection process can be
increased, so that a reduction in the spatial resolution of the
reconstructed image can be suppressed.

[0074] Further, when the interval between the X-ray paths at the
equally-spaced parallel beam projection data P3 in the channel direction
is set to 1/N (where N is an integer greater than or equal to 2) of the
reference interval Δd, data themselves at the unequally-spaced
parallel beam projection data P1 or post-interpolation data close thereto
can be more included in the equally-spaced parallel beam projection data
P3, thus making it possible to more suppress a reduction in the spatial
resolution of the reconstructed image.

[0075] Incidentally, the larger the integer N, the more the effect of
suppressing the reduction in the spatial resolution increases. The effect
however gradually maxes out, whereas the amount of computational
processing continues to increase. Therefore, when the balance between the
effect and the amount of calculation is taken into consideration, for
example, N may be set in a range from about 2 to 4.

[0077] When multidimensional interpolation is used as the interpolation
process, the order of the interpolation process may be changed according
to the distance of each X-ray path corresponding to data attempted to be
acquired by this interpolation process from the iso-center ISO. For
example, when the distance decreases, the order of the interpolation
process may be set to decrease. As the distance increases, the order of
the interpolation process may be set to increase. If done in this manner,
an interpolation process more appropriate to high and low tendencies of
the spatial resolution as seen in the radial direction from the center
corresponding to the iso-center ISO in the reconstructed image can be
applied, and hence a reduction in the spatial resolution can be expected
to be further suppressed.

[0078] Assigning weights to the original data used in the interpolation
process may be changed according to the distance between each X-ray path
corresponding to data attempted to be obtained by this interpolation
process and the X-ray path corresponding to the original data used in
this interpolation process. That is, nonlinearity may be applied to the
weighting. For example, when the distance decreases, the weight may be
made large, whereas as the distance increases, the weight may be made
small. If done in this manner, when the X-ray path corresponding to the
post-interpolation data can be determined to be enough close to the X-ray
path corresponding to the original data used in the interpolation
process, the weight to be assigned to the original data can be made
larger as compared with the linear interpolation, whereby data closer to
actual data can be obtained and a reduction in the spatial resolution can
be expected to be further suppressed.

[0079] Incidentally, the rearrangement process and the interpolation
process in the channel direction may respectively be performed on the
algorithm stepwise in parts or may be performed collectively in the form
of single processing.

[0080] The back-projection processing unit 33 performs a back-projection
process on the equally-spaced parallel beam projection data in the plural
view directions acquired by the fan-para conversion unit 32 to
reconstruct an image. As the back-projection process, there can be used,
for example, a filtered back-projection process, a convolution
back-projection process, etc. The filtered back-projection process is of
a process which multiplies Fourier transformation of projection data by a
reconstruction function (filter function) in a frequency space and
performing inverse Fourier transformation to reconstruct an image. The
convolution back-projection process is of a process which determines
inverse Fourier transformation of a reconstruction function and overlays
it on projection data on a real space, i.e., performs convolution on it,
followed by being back-projected, thereby reconstructing an image.

[0081] The flow of an image generating process in the X-ray CT apparatus
according to the exemplary embodiment will be explained.

[0082]FIG. 8 is a flow chart showing the flow of the image generating
process in the X-ray CT apparatus according to the exemplary embodiment.

[0083] At Step 51, the data acquisition unit 31 runs a scan to acquire fan
beam projection data of plural views. At this time, for example, a scan
is done while assigning views for acquiring actual data to a rotating
angle corresponding to one rotation of scan for every rotating angle
corresponding to the interval of arrangement of detecting elements 24i.
Incidentally, when fan beam projection data of view numbers corresponding
to it are generated inclusive of interpolation in each view direction by
the actual data, views for performing acquisition of the actual data are
assigned at least 1200 or more to the rotating angle corresponding to one
rotation of scan.

[0084] At Step S2, the fan-para conversion unit 32 performs a
rearrangement process and an interpolation process in a channel direction
on the fan beam projection data of the plural views acquired at Step S1
to thereby perform fan-para conversion, thereby acquiring equally-spaced
parallel beam projection data. At this time, the interval between X-ray
paths in the channel direction is set to an interval equivalent to 1/N
(where N is an integer ranging from 2 to 4, for example) of the reference
interval Δd.

[0085] At Step S3, the back-projection process is performed on the
equally-spaced parallel beam projection data acquired at Step S2 to
reconstruct an image.

[0086] A description will now be made of a result of comparison between a
reconstructed image by a generally-used method (hereinafter called
"general method") and a reconstructed image by a method (hereinafter
called "exemplary method") based on the exemplary embodiment.

[0087]FIG. 9 is a diagram showing a first example of comparison between
an image by the general method and an image by the exemplary method. The
first example is an example in which a phantom for MTF measurement is
placed so as to assume a position where its center is spaced a radius 50
mm from an iso-center ISO and is scanned. An image G11 (Original) on the
left side is an original image based on the general method and acquired
when the interval between X-ray paths at the interpolation process as
seen in the channel direction is set to the reference interval Δd.
An image G12 (chup2) on the right side is an image based on the present
method and obtained when the interval between X-ray paths at the
interpolation process as seen in the channel direction is set to 1/2 of
the reference interval Δd, i.e., sampling in the channel direction
is set to be as double as dense. An image G13 on the lower center side is
an image corresponding to a difference between these. As shown in FIG. 9,
in the original image G11 based on the general method, a pin in the
phantom for the MTF measurement extends in the channel direction (the
radial direction centering on the iso-center). On the other hand, it is
understood that in the image G12 based on the exemplary method, its pin
is represented as the original circular shape and has been improved.

[0088] FIG. 10 is a diagram showing a second example of comparison between
an image based on the general method and an image based on the exemplary
method. The second example is an example in which the same MTF
measurement phantom as that shown in the first comparative example is
placed so as to assume or take a position where its center is spaced a
radius 100 mm from an iso-center ISO and is scanned. An image G21
(Original) on the left side is an original image (interval between X-ray
paths="reference interval") based on the general method. An image G22
(chup2) on the right side is an image (interval between X-ray paths =1/2
of "reference interval") based on the exemplary method. An image G23 on
the lower center side is an image corresponding to a difference between
these. A graph 24 lateral to the image G23 is obtained by plotting line
pairs per cm(lp/cm) at the time that the MTF value becomes 10%, and line
pairs per cm(lp/cm) at that time that the MTF value becomes 50%, with
respect to the distance from the iso-center ISO. In the original image
G21 based on the general method, as is understood even from the
differential image shown in FIG. 10, a Sharp/Blur band has occurred over
the circumferential direction according to the distance from the
iso-center ISO. On the other hand, the image G22 based on the exemplary
method is improved to be an image in which the occurrence of such a band
is suppressed and its spatial resolution is flat.

[0089] FIG. 11 is a diagram showing a third example of comparison between
an image based on the general method and an image based on the exemplary
method. The third example is an example in which a head phantom is
scanned. Its image indicates the structure of a bone region of the inner
ear lying in a position spaced a radius 85 mm from an iso-center. An
image G31 on the upper left side is an original image based on the
general method and obtained when the interval between each X-ray path at
the interpolation process in the channel direction is set to the
reference interval Δd. An image G32 (chup4) on the upper right side
is an image based on the exemplary method and obtained when the interval
between each X-ray path at the interpolation process in the channel
direction is set to 1/4 of the "reference interval", i.e., sampling in
the channel direction is set to be four times as dense. An image G33 on
the lower left side is an enlarged view of a region surrounded by a
broken line in the image G31. An image G34 (ch2v2) on the lower right
side is an enlarged view of a region surrounded by a broken line in the
image on the upper right side. It is understood that even at a region
away from the iso-center ISO, its spatial resolution has been improved in
the image based on the exemplary method.

[0090]FIG. 12 is a diagram showing a fourth example of comparison between
an image based on the general method and an image based on the exemplary
method. The fourth example is an example in which a head phantom is
scanned. Its image indicates the structure of a local region of the bone
of the inner ear. An image G41 (Original) on the upper left side is an
original image based on the general method and obtained when the interval
between each X-ray path at the interpolation process in the channel
direction is set to the reference interval Δd. An image G42 (chup2)
on the upper right side is an image based on the exemplary method and
obtained when the interval between each X-ray path at the interpolation
process in the channel direction is set to 1/2 of the reference interval
Δd, i.e., sampling in the channel direction is set to be twice as
dense. An image G43 (v2) on the lower left side is one obtained when the
number of views for performing the acquisition of actual data is set to
twice (1600 or more per rotation) the normal number, i.e., sampling in
the view direction is set to be twice as dense. An image G44 (ch2v2) on
the lower right side is one obtained when sampling in both of the channel
and view directions are set to be twice as dense as normal. As is
understood from FIG. 12, the image obtained when the sampling in both
channel and view directions are set to be twice as dense as normal is
highest in spatial resolution.

[0091] Thus, according to the exemplary embodiment, the equally-spaced
parallel beam projection data are acquired in such a manner that when the
fan-para conversion is performed, the interval between each radiation
path in the channel direction becomes the interval smaller than the
reference interval obtained by dividing the interval of arrangement of
the detecting elements in the channel direction by the projection
enlargement rate at the detection surface of the detector taken when the
so-called iso-center is set as the reference. Therefore, the
pre-interpolation data (i.e., data before the interpolation processing)
high in accuracy or the post-interpolation data (i.e., data after the
interpolation processing) close thereto can be more used in the
back-projection process. Even if the fan-para conversion is performed, a
reduction in the spatial resolution of the reconstructed image can be
suppressed.

[0092] Incidentally, various changes and additions and the like can be
made to the exemplary embodiment without departing from the scope or
spirit of the disclosure.

[0093] For example, although the exemplary embodiment is implemented using
an X-ray CT apparatus, an image generating apparatus that performs the
above image generating process is also one example illustrative of an
embodiment of the invention. A program for allowing a computer to
function as such an image generating apparatus, a storage medium in which
the program has been stored, or the like is also one example illustrative
of an embodiment of the invention.

[0094] For example as well, although the exemplary embodiment is
implemented using an X-ray CT apparatus, the disclosure is applicable
even to a PET-CT apparatus or SPECT-CT apparatus in which the X-ray CT
apparatus and PET or SPECT are combined together, a general imaging
apparatus, etc.